Radar systems or general distance sensors which are mounted in or on a motor vehicle and are utilized, for example, in the context of a distance warning system or an adaptive vehicle speed control system, are now known from numerous publications, for example, International Patent Application No. WO 91/09323 ("the document"). Such systems can, for example as in the document, be implemented as microwave systems, or alternatively or additionally can be based on light waves. The document proposes a radar system having an antenna arrangement whose opening angle is preferably variable as a function of the vehicle's speed. According to the document, the need for such a feature arises from the fact that the observation region of such a radar system, and thus the opening angle of its antenna arrangement, should be broader in city traffic (and thus at lower speeds) than, for example, on an expressway. The disadvantage of a generally very broad opening angle lies, according to the document, in the higher costs associated therewith. The disadvantage of too narrow an opening angle lies in the fact that, in particular, targets which are located at a short distance laterally from the distance sensor cannot be detected and are thus overlooked. The arrangement described in the document allows alternation between a broad and a narrow observation region, but the two can only be set alternatively to one another in each case. It is a further disadvantage the fact that the manner in which the object of the document is achieved requires that changes be made to the antenna arrangement itself during operation of the radar system. Since, however, the accuracy requirements for adjustment of such an antenna arrangement are extremely high in the frequency range used (currently about 77 GHz), each action upon or modification to the antenna arrangement may be expected to cause degradation from an optimum setting.
U.S. Pat. No. 4,769,646 describes an antenna system having an antenna lens that, in combination with two feed elements, simultaneously constitutes a broad and a narrow antenna beam. A broader spatial or angular region is observable with the broad antenna beam, and a narrow spatial or angular region with the narrow antenna beam. The antenna arrangement is carried out in that the antenna lens has multiple lens regions which are configured so that electromagnetic waves are more strongly, more weakly concentrated or scattered as they pass through the respective lens region. With the arrangement described here, however, the possibility cannot be ruled out that signal components which derive from target reflections from the broader observation region of the broader antenna beam will also be received by the feed element which is provided in order to constitute the narrow antenna beam, and vice versa. This necessarily leads to misinterpretations in the assessment of the environmental situation received by the radar system. In technical terms, the two feed elements with their different observation regions are not completely decoupled from one another.